NMOS LDO Regulator Stability with Auxiliary Amplifier
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Solution Overview
Problem
NMOS LDO regulators face instability and reliability issues due to the use of high ESR capacitors, which compromise transient performance and cause joule heating problems, while low ESR ceramic capacitors are not effectively supported by existing technologies for stable operation and high phase margin.
Innovation Solution
The introduction of a parallel AC feedback path and an auxiliary amplifier loop that takes control at high frequencies, tracking the transfer characteristic pole associated with the load capacitance to provide a predetermined phase margin, ensuring stability across a wide range of ESR and capacitance values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If high ESR output capacitors are used to provide a zero to cancel the second pole, then stability is improved, but transient performance deteriorates and joule heating reliability problems occur
Solution Approach 1:
The feedback path is segmented into two separate paths: a DC feedback path for low-frequency stability and an AC feedback path for high-frequency stability. This segmentation allows each path to be optimized independently, enabling the use of low ESR capacitors without compromising overall circuit stability.
Solution Approach 2:
An auxiliary amplifier is introduced as an intermediary component to implement the AC feedback path. This mediator processes the AC feedback signal separately and combines it with the DC feedback, enabling stable operation with low ESR capacitors by providing the necessary phase compensation without requiring high ESR.
2Stability of the object's composition
If high ESR output capacitors are used to cancel the second pole, then stability is improved, but transient performance deteriorates
Solution Approach 1:
The feedback system is divided into DC and AC paths that operate at different frequency ranges. The AC feedback path specifically addresses high-frequency transient responses, allowing fast transient performance while the DC path maintains overall stability, eliminating the need to sacrifice transient speed for stability.
Solution Approach 2:
An AC feedback path is implemented that provides frequency-dependent feedback. This feedback mechanism automatically adjusts the loop gain at high frequencies to maintain stability during transient conditions, enabling both fast transient response and circuit stability simultaneously.
3Stability of the object's composition
If a parallel AC feedback path is added to improve stability with low ESR capacitors, then stability is improved, but device complexity increases
Solution Approach 1:
The AC feedback path is merged with the existing DC feedback path through the auxiliary amplifier output. Both feedback paths share common components such as the error amplifier input stage and the pass transistor control node, reducing the overall component count and simplifying implementation despite the dual-path architecture.
Solution Approach 2:
The auxiliary amplifier is designed to perform multiple functions: it processes the AC feedback signal, provides phase compensation, and interfaces with the existing DC feedback path. This multi-functionality reduces the need for separate dedicated components, thereby limiting the increase in device complexity while achieving the desired stability improvement.
Data Source
AI summary
An LDO (low dropout) regulator including a pass transistor having a first electrode coupled to produce an output voltage of the LDO regulator, a control electrode, and a second electrode coupled to receive an input voltage of the LDO regulator. An error amplifier has a first input coupled to a first reference voltage and an output coupled to the gate control electrode of the pass transistor. A first feedback circuit has an input coupled to the first electrode of the pass transistor and an output producing a first feedback voltage coupled to a second input of the error amplifier. The auxiliary amplifier has a first input coupled to a second reference voltage and an output coupled to the output of the error amplifier. A second feedback circuit has an input coupled to the output of the auxiliary amplifier and an output producing a second feedback voltage coupled to a second input of the auxiliary amplifier. The auxiliary feedback loop is used to take over control of the feedback in the LDO regulator at high frequencies.